The present invention is directed to a weld motor control system. More particularly, the present invention is directed to a weld motor control system for a strapping tool or strapping machine that provides a consistent weld or joint strength regardless of weld motor voltage fluctuations.
Strapping tools or machines are well known in the art. Typically these machines are used for securing straps around loads. One type of strapper is used with a plastic or polymeric strap and is a stationary arrangement in which the strapper is included as part of an overall manufacturing or packaging system.
A strapping or welding head is provided as part of a strapping machine and provides a number of functions. First, the strapping head includes a gripper having one or more gripper portions that grip the strap during the course of a strapping operation. The strapping head also includes a cutter to cut the strap from a strap source or supply. Last, the strapping head includes a sealer to seal an overlying course of strapping material onto itself. This seal is commonly referred to as a weld and is effected by heating overlying courses of the strap by use of a vibrating element.
To effect the seal or weld, an anvil is maintained rigidly against one of the courses of strap and a vibrating element oscillates or vibrates against the other course of strap, thus creating friction and heat to effect the weld. The vibrating element is driven by a motor that is mounted to the body and operably connect to the vibrating element. In a typical weld motor arrangement, the weld cycle is controlled by time. That is, the weld cycle is a time dependent cycle, typically timed or controlled by either an electric timing circuit or a pneumatic timing circuit.
It has, however, been found that the weld motor voltage can decrease during the weld cycle as much as 3 percent to 4 percent in addition to a 10 volt fluctuation in the power feed, commonly experienced. Moreover, this voltage decrease has been correlated to lower joint strength because the motor speed decreases as the voltage decreases. In an effort overcome the lower joint strength, longer weld times were used. However, it was found that when longer weld times were used in a cycle in which the voltage did not drop, the strap integrity could be, in certain instances, compromised.
Accordingly, there exists a need for a simplified weld motor control system for a welding or strapping head for use in a strapping machine. Desirably, such a control system eliminates the dependence upon time as the controlling factor for the weld cycle. More desirably, such a control system serves to provide a consistent strap weld, regardless of fluctuations in the weld motor voltage. Most desirably, such a control system is readily adapted to existing strapping machine strapping head systems.